MOSCOW — Alexei A. Navalny, Russia’s leading opposition figure, was placed under house arrest on Friday and ordered not to use the Internet or telephone for two months, thus removing President Vladimir Putin’s fiercest critic from public life.

In his verdict, Judge Artur Karpov of Basmany Court in Moscow ruled that Navalny had violated the terms of a travel ban from a pending criminal case accusing him of defrauding a local branch of cosmetics producer Yves Rocher of more than $500,000.

The stiff restrictions in what is widely seen as a politically charged prosecution will effectively muzzle Navalny, the blogger-turned-politician who has used social media to trumpet mass demonstrations against the Kremlin and release damning profiles of corrupt practices in government bids, the most recent asserting that billions of dollars were stolen in the preparations for the Sochi Olympics.

“Their only goal is to stop my political activities,” Navalny told Karpov in front of a packed courtroom. “They want to stop me from coordinating our anti-corruption investigations.”

The ruling, which also prohibits Navalny from speaking with the news media or accepting visitors other than close family members, capped a week in which the Russian authorities showed a renewed will to disrupt demonstrations, detain large numbers of people, and hand down tough sentences to curtail internal dissent since the conclusion of the Winter Games on Sunday.

In the months before the games, Putin gave amnesty to several of Russia’s most prominent prisoners, including a former oligarch, Mikhail B. Khodorkovsky, and two members of the punk protest group Pussy Riot, in what was seen as a nod to international criticism of their prosecutions.

With the games over, the Kremlin has taken its most decisive step yet to silence Navalny as it continues to dampen the mood of protest that erupted two years ago.

“It is easy to see that with the Olympics over, there’s no need to put up a kind face for anyone anymore,” Sergei Nikitin, the head of the Russian branch of Amnesty International, said in a telephone interview Friday. “We are all witnesses to Russia’s growing pressure on any kind of independent opinion.”

On Monday, a judge handed down sentences of two to four years in prison camps for seven activists detained during a 2012 anti-Putin rally that deteriorated into clashes between demonstrators and police officers, a case that Nikitin called “a parody of the administration of justice.” In a rally that followed, the police detained more than 400 peaceful protesters, including Navalny, who was subsequently convicted of resisting arrest and sentenced to seven days in jail.

The fraud case Karpov ruled on Friday is one of several criminal prosecutions brought against Navalny that seem politically motivated and largely trumped-up to give the authorities ways to curtail his movements and communication and to silence his criticism of Putin.

Prosecutors requested house arrest after Navalny was detained at Monday’s rally, claiming that he was disrupting public order at the demonstration and that he had traveled to the suburbs of Moscow in early January.

“It’s a travel ban,” Karpov told Navalny, who stood in boots without laces, which were removed because he has been held in jail since Monday. “It meant you couldn’t go where you were not given express permission.”

Navalny has often skirted on the edge of prison, and his fate has been seen as a barometer of state pressure on the opposition in Russia.

Navalny was convicted in July in a separate embezzlement case and sentenced to five years in prison. He was freed the next day on appeal and allowed to run for mayor of Moscow. At the time, his candidacy seemed to suit the Kremlin’s political goals by helping to portray the election as a genuine and hard-fought victory by the incumbent, Sergei S. Sobyanin, an ally of Putin’s.

A State Department report released Friday criticized Russia for Navalny’s prosecution, citing it as one where “officials denied due process in politically motivated cases initiated by the Investigative Committee,” the law enforcement body that asked that Navalny be placed under house arrest.

Anna Veduta, Navalny’s aide, said outside the courthouse that Navalny’s anticorruption organization had divided his responsibilities and social media accounts and would continue working, although his colleagues were barred from communicating with him.

Jocular and irreverent, Navalny railed against prosecutors during the nearly two-hour trial, calling the terms of his travel ban “absurd.”

A technophile who is rarely without an iPhone, he requested a computer from Veduta, saying “it may be my last hours” to use the Internet.

“I have a fairly popular blog,” Navalny told the court during oral arguments. “Two million people read it a month.”

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